Understanding the exchange between seepage and overlying surface water has become increasingly important due to the potential impacts to the environment resulting from anthropogenic land uses. The key input for submarine groundwater discharge (SGD) in near-shore environments is believed to be the discharge from land to surface water induced by the hydraulic gradient in the terrestrial aquifer. However, significant contribution to SGD may also derive from groundwater circulation and oscillating flow induced by tidal stage as well as salinity and thermal variations. This discharge carries with it contaminants and/or nutrients, dissolved and/or colloidal, that have the potential to impact the chemical budget of surface water ecosystems. This impact, along with other biological and physical impacts, may be heightened in smaller bodies of water such as embayments or lagoons due to their limited volume and restricted fluid exchange with the open ocean.
A major obstacle in studying SGD is accurately measuring groundwater seepage across the sediment-water interface. Discharge rates may be as low as <1 cm/day and these low rates make quantification of SGD inherently difficult. In addition, the ebullition of gas from sediments is a common event, further increasing the difficulty of accurately measuring SGD.
Current methodologies for measuring SGD have included a system that utilizes a 4-liter plastic collection bag and a cut off section of a 55 gallon drum as described by D. R. Lee, in “A Device for Measuring Seepage Flux in Lakes and Estuaries,” Limnology and Oceanography, 22: 140–147, 1977. Using this device, the open-ended section of a cut off section of a 55-gallon drum is inserted into the sediment. Attached to the drum via an outflow port is a 4-liter plastic bag that collects the seepage. The volume of the bag and sampling interval are recorded and the specific discharge velocity is obtained by dividing the volume of collected seepage over the time interval by the area of the drum. Although this method can be effective, various errors have been associated with the device that must be corrected for prior to sampling. Another disadvantage to this method is that it is quite labor intensive since the plastic bags need to be monitored and replaced continuously. In addition, data collected are averages over the specified time interval and may not fully quantify short term events. Furthermore, this method is incapable o measuring reverse flow.
Continuous logging seepage meters have been developed utilizing heat-pulse technology as described by M. Taniguchi and Y. Fukuo, in “Continuous Measurements of Ground-Water Seepage Using an Automatic Seepage Meter,” Ground Water, 31, no. 4: 675–679, 1993. This method, however, cannot be monitored during deployment and may therefore malfunction during the collection period without notice. Another disadvantage of this method is that it cannot measure seepage in intertidal environments in which the seepage meter becomes periodically air bound during low tide events. In addition, variations in the water density and temperature can also affect the accuracy of the heat pulse method.
Piezometric head measurements have also be used to estimate the specific discharge of groundwater to surface waters. This method requires the installation of monitoring wells offshore to monitor the hydraulic head beneath the surface water. The method can determine if water is entering or exiting the surface water but in order to determine the specific discharge estimates of the hydraulic conductivity of the sediment are needed. However, this method is not a direct measurement of seepage but and estimate based on head measurements and sediment conductivity.
Accordingly, there is a need for a remotely-deployable device capable of accurately measuring SGD in both the forward and reverse flow directions.